1. Field of the Invention
The present invention relates to a toner for use in electrophotography, and an image forming method using the toner.
2. Discussion of the Background
Electrophotographic image formation is typically performed by the following series of processes as described in U.S. Pat. No. 2,297,691:
(1) Forming a latent electrostatic image on a photoreceptor, i.e., a latent electrostatic image bearing member;
(2) Visualizing the latent electrostatic image with developer to form a visualized image (toner image); and
(3) Transferring the visualized image on a recording medium such as paper; and
(4) Fixing the transferred image to obtain a fixed image.
Conventional developers include single component developers using a magnetic toner or a non-magnetic toner, and double-component developers containing toner and carrier.
As for the fixing method mentioned above, a heat roller system is widely used in which a heat roller is directly contact-pressed to a toner image on a recording medium in terms of energy efficiency. However, when such a heat roller system is used, there is a drawback in that a large amount of electric power is required for the fixing. Therefore, in attempts to save energy, reducing the consumption of power by the heat roller has been variously studied. For example, there is a widely used system in which the power of a heat roller is reduced when no image is output and is increased when images are output. However, such a system requires several tens of seconds waiting time from a sleeping time before the temperature of the heat roller is raised enough for fixing. This waiting time is a stressful time for a user. In addition, when images are not output, it is demanded that the heater for a heat roller should be completely off for further restraint on power consumption. To satisfy these demands, it is preferred to obtain a toner having a low fixing temperature.
To obtain such toner having a low fixing temperature, it is desired to control the thermal characteristics of a resin in the toner. For example, a method has been proposed in which the glass transition temperature (Tg) of a resin is lowered by adding a melting-mixable material (hereinafter referred to as plasticizer) having a plasticizing effect. However, when Tg is excessively lowered, thermal preservability deteriorates. When the 1/2 temperature (F1/2) in the flow tester method is excessively lowered, a drawback arises such that the hot offset occurrence temperature is lowered. Therefore, it has been a long-standing challenge to develop a toner having a good combination of low temperature fixability and heat resistant property, which have a trade-off relationship.
To achieve such a good combination of low temperature fixability and heat-resistant preservability, for example, published unexamined Japanese patent application No. (hereinafter referred to as JOP) H06-258861 describes a toner in which a resin without containing a plasticizer protrudes from the surface of the toner particle. The inside of the toner particle having such resin protruding therefrom is mainly formed of a resin having a low Tg. Therefore, it is difficult to provide such a toner with a good combination of low temperature fixability and heat resistance property.
For example, JOP 2002-221825 describes toner containing a binder resin and a wax having a melting point of 20 to 150° C. When the binder resin is thermally dissolved in the wax functioning as a solvent followed by rapid cooling down, the binder resin is not separated and precipitated in the wax. In the toner, the wax has a plasticizing effect for a resin having a melting point higher than the fixing temperature. Therefore, the toner has a good low temperature property. However, the toner is manufactured by a pulverization method including a melting and kneading process. Thereby, the toner already has a plasticizing effect when manufactured, resulting in insufficiency of heat-resistant property.
JOP 2002-202627 describes toner containing a resin and two kinds of waxes. One is compatible with the resin monomer and the other is not compatible therewith. However, the toner described in JOP 2002-202627 is mainly for a single-component developer and its object is to uniformly disperse a magnetic substance in the toner. The plasticizing effect of the wax compatible with the toner is a supplemental effect in comparison to the dispersion of the magnetic substance. In addition, in the process of manufacturing the toner, there is a heating process in which the temperature is higher than the melting point of the compatible wax. Therefore, as in the toner described in JOP 2002-221825, the toner already shows a plasticizing effect when manufactured, which leads to a problem that the heat-resistant property deteriorates.
JOP 2001-281909 describes toner containing a multi-functional ester compound compatible with the resin monomer. However, it is just that the resin monomer used in the toner has a melting point close to the melting point of the multi-functional ester compound. The glass transition temperature (Tg) of the toner means that the toner is in the state in which the plasticizing effect of the multi-functional ester compound is already shown. Therefore, it is also impossible for the toner to sufficiently obtain a good combination of the low temperature fixability and the heat-resistant property.
To obtain a good combination of the low temperature fixability and the heat-resistant property, for example, microcapsule toner has been proposed which has a shell portion formed of a compound having a high melting point and a core portion formed of a coloring phase which is liquid at room temperature. As an example of such toner, JOP H06-19182 describes a microcapsule toner having a micro phase separation structure formed of a dispersion phase and a continuous phase and having a copolymer compatible with both phases as a core portion to improve stability of the image after fixing. However, in the toner having such a structure, it is still necessary to apply pressure on fixing. In addition, such toner still has such problems as stability of the toner, disturbance of the image, and deterioration of gloss. Therefore, it is still impossible to obtain characteristics desirable as toner.